Method of limiting the speed of constant-current series motors.



No. 768,468. PATENTED AUG. 23, 1904.

M. LEBLANG.

METHOD OF LIMITING THE SPEED OF CONSTANT CURRENT SERIES MOTORS.

APPLICATION FILED P5126, 1903. N0 MODELI WITNESSES 11v VEN TOR C i )l'iklum ce Z6Zanq Allomzys UNITED STATES Patented August 23, 1904.

PATENT OEEIcE.

MAURICE LEBLANC, OF PARIS, FRANCE, ASSIGNOR TO WESTINGHOUSE ELECTRIC AND MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA.

METHOD OF LIMITING THE SPEED OF CONSTANT-CURRENT SERIES MOTORS.

SPECIFICATION forming part of Letters Patent No. 768,468, dated August 23, 1904.

Application filed February 26, 1903. Serial No, 145,283. (No model.)

To all whom it may concern:

Be it known that I,MAURIoE LEBLANo, a citizen of the Republic of France, residing at Villa Montmorency, No. 1 Avenue de Bouffleurs, Paris, France, have invented a new and useful Method of Limiting the Speed of Constant-Current Series Motors, of which the following is a specification.

The object of this invention is to regulate or limit the extreme speeds of a motor whose field has a series winding and which motor is subjected to the action of a direct current of constant strength. Now itis well known that if we disregard armature reactions such a motor under such conditions has no definite speed for a given load. It is as likely to run fast and even to race as to run slow; but if we take into account armature reactions we find there is a certain permissible maximum load which the motor may carry. If greater load than this maximum load is placed upon the motor, it will not start. If the maximum load or a load very nearly equal to the maximum load is placed upon the motor, the motor will start and run at slow speed. If this load be now diminished, the motor speeds up until the hurtful electric reactions, the eddy-currents, and the mechanical friction of the motor at a greater speed added to the diminished load on the motor produces a new state of equilibrium at some higher speed. At a load considerably below the maximum the motor will run at a very high speed indeed, which means that it will race. Such automatic regulation of a series-wound motor supplied by a direct current of constant strength as has just been discussed is naturally very inefiicient.

It is the object of my invention to so vary with relation to its velocity the field of a serieswound motor fed with a constant direct current that the maximum velocity which the motor may have is limited to such a predetermined maximum speed as I may choose to select, no matter how small the load. In fact, in accordance with my method the speed of the motor under varying loads is not far from constant. To this end I counteract the effect of the series winding of the motor upon the motor-field by means of a current which is either of zero value or small until the maximum desired speed of the motor is approached and which then rather suddenly assumes a value suflicient to practically neutralize the effect of the series winding of the motor upon the motor-field, so that the maximum desired speed may never be exceeded. Such a current I may produce in various ways, and I have shown as one means of producing it a dynamo arranged to revolve at a speed which shall be uniform with relation to the speed of the motor. Thus I may mount the dynamo upon the shaft of the motor so as to insure that when the motor runs slowly the dynamo shall run slowly and when the motor runs fast the dynamo shall run correspondingly fast. I thereupon so design this dynamo and electrically connect it to the motor in such a manner that at slow speeds of the motor, and therefore slow speeds of the dynamo, the dynamo shall furnish practically no current to the motorfield opposing the effect of its series winding; but as the motor reaches its selected maximum speed, whether with small or large loads, the now more rapidly revolving dynamo shall quite suddenly furnish sufiicient opposing current to the motor-field to practically neutralize the motor-field. In this way it is seen that the speed of the motor can never exceed the selected maximum, for when the motor reaches this maximum speed the dynamo, which now revolves rapidly, suddenly generates an opposing current sufiicient to practically wipe out the motor-field, which means that the motor must slow down.

The dynamo should be self-exciting. Its design will be such that the magnetization of its field will not build up to any appreciable extent at slow speeds, but that it will build up quite rapidly at a speed corresponding to the preselected maximum speed of the motor. Furthermore, it is desirable that the field of this dynamo should have very little residual magnetism and that its armature-circuit be closed in derivation upon the series winding of the motor-field, so that the field. of the dynamo may always have an initial, though small, excitation, from which its field can be built up when the proper velocity has been reached without relying upon any residual magnetism in the dynamo.

In the drawings, Figure 1 shows a diagram .of a main circuit containing motors and regu lating-dynamos connected therewith, and Fig. 2- shows the characteristics of the regulatingdynamo.

The main dynamo K, which is indicated conventionally, feeds a main circuit XY with a direct current of constant strength. In this main circuit are placedone or more directcurrent motors A, having commutators A and brushes A, which brushes are in series with the field-winding B of the motor.

As I have stated before, I may generate in difi erent ways by different apparatuses the current which, as the maximum desired speed of the series-wound motor is approached, somewhat suddenly assumes a strength sufiicient to practically neutralize the field magnetization produced by the main current of constant strength passing through the series field-winding of the motor; but to show one way of practically executing the method which is the subject of this application I have represented a small self-exciting dynamo D, which is moun ted upon the motor-shaft or arranged with re spect to the motor in any other suitable way to produce a constant relative speed. I have inclicated a series-wound dynamo the series fieldcoil E of which is mounted in series with the brushes D which bear upon the commutator D of the dynamo D. The armature-circuit of this dynamo D supplies a winding C, which is mounted upon the field of the motor A in opposition to its field-winding B. This opposed winding C, I usually make of high resistancethat is, of fine wire and I further prefer to mount the external circuit of the dynamo D, which contains the opposed winding C, in derivation of the field-winding B of the motor or in derivation of a part of this winding. In this manner I secure the presence of a small exciting-current in the fieldwinding E of the dynamo D without having to rely on any residual magnetism in the dynamo, which residual magnetism it is of advantage to make as nearly equal to zero as possible. I also arrange the connections so that the current which initially passes through the coil C as a branch of the main-line current is opposed in magnetic effect to the current in the coil B. Nevertheless the current which traverses the opposed winding 0 will be small at low speeds of the motor, and therefore of the dynamo, so that at such speeds the counter-magnetizing force due to the opposed winding 0 will be negligible with respect to the magnetizing force of the series windings B of the motor. The strength of the field of the motor at slow speeds will thus not be sensibly diminished by the presence of the dynamo D and the opposed winding C.

As the motor and in consequence the dy namo increase in speed the strength of the current in the winding 0 will increase; but it will increase very slowly at first and then very suddenly, when the velocity of the dynamo has become sufficient to permit it to build up its excitation. The dynamo is so designed that this last-mentioned velocity shall correspond to the maximum velocity which it is desired that the motor shall have and that the current generated by the dynamo at this velocity shall be sufficient when traversing the opposed winding C to neutralize the magnetizing efi'ect of the series winding B of the motor, and therefore to wipe out the magnetic field of the motor. It follows at once that the motor can never attain a greater speed whether for small or for large loads than such as corresponds to that speed of the dynamo at which its field excitation has built up sufficiently for it to produce such a current in the opposed winding C of the motor as will neutralize the field of the motor. It equally follows that a comparatively small diminution of the velocity of the motor, and therefore of the dynamo, below this maximum speed will, if its residual magnetism is-negligible, bring about the withdrawal of the building-up action in the dynamo-field, so that the dynamo will no longer send an appreciable current into the opposed winding C of the motor.

I have already remarked that it is desirable to reduce the residual magnetism of the regulating-dynamo to the lowest possible point.

I may further remark that it is desirable to thoroughly laminate the fields of the motor and of the regulating-dynamo to render their magnetic circuits very permeable in reducing the air-space to a minimum and not to saturate the iron.

I come now to an explanation of the characteristic curve of the regulator-dynamo with relation to the problem in hand. Referring to Fig. 2, I have shown a set of rectangular coordinates, the horizontal axis 02 being taken to measure the current strength-and the vertical axis ()0 being taken to represent electrical pressure. It is well known that if a series-wound dynamo is run at a certain constant speed IV the curve which represents the relation between current strength and electrical pressure under varying resistances in the current is of the general shape represented at OMVV. I have drawn the straight line OR such that the tangent of the angle ROI: shall beequal to the total resistance in the armature-circuit of my regulator-dynamo. Under such assumptions it is explained in the text-books that when the regulator-dynamo is running at a speed WV to supply a circuit of total resistance R the regulator-dynamo will give off a current proportional to the length OI under an electrical pressure proportional to the length IM. My regulator-dynamo must therefore be designed so that the length OI corresponds to the strength of current necessary to send into the opposed winding 0 of the motor A to completely neutralize the field of the motor or the effect of its series winding B at what now becomes a preselected maximum speed IV, being the speed which the motor is never to exceed. If now I further design my regulator-dynamo so that the portion of its characteristic curve OM for the maximum speed W very nearly coincides with the straightline OR, it will be found that the characteristic of the dynamo for a speed IV, but little less than the maximum speed WV, will fall almost entirely below the straight line OR and will intersect this straight line in a point P very near the origin of coordinates. This means that at the arbitrarily selected speed WV, somewhat less than the maximum permitted speed IV, the amount of current which the dynamo under the given assumption sends into the opposed winding C of the motor will be proportional to the length OJ-that is to say, it will be very small. Knowing the amount of this opposing current, we can readily deduce therefrom the intensity of the motor-field and in consequence the value K of the couple developed upon its axis when it turns with the velocity V. This couple K will be somewhat though but little less than the theoretical maximum couple K, which the motor has at starting or zero speed.

So long as we do not put a greater load upon the motor than corresponds to the couple K the speed of the motor will not sink below the speed WV. Since this speed W need be taken but little below the selected maximum speed IV, as the characteristic curves on Fig. 2 show, it follows that so long as my motor is not required to overcome a resisting-couple greater than K, which is but little less than the theoretical maximum couple K, which the motor can stand, the speed of my motor will vary only between the limits W and W, and these may be taken quite near together, as Fig. 2 shows. This means that my motor operates under practically constant speed from zero load up to that represented by the couple K.

Vhile I have spoken of completely neutralizing the motor-field at the maximum permitted velocity, it will be understood that in actual practice it may not ordinarily be necessary to completely neutralize this field.

From what has been said it will appear that the method which is the subject of this application consists in limiting the maximum speed of a series-wound direct-current motor supplied with line-current of constant strength for all loads by practically neutralizing the magnetizing effect of the line-current upon the motor as the maximum desired speed is approached. More specifically stated, this method comprises the sudden generation of a neutralizing-current as the maximum desired speed is approached. To limit the minimum speed of the motor, I use the expedient of not placing upon the motor a greater maximum load than it can bear at the minimum desired speed.

I claim- I. The method of limiting the speed of a series-wound direct-current motor, which consists in feeding it with a direct current of constant strength, and practically neutralizing the magnetizing effect of the line-current upon the motor at a given maximum speed predetermined for all loads, substantially as described.

2. The method of limiting the speed of a series-wound direct-current motor, which consists in feeding it with a direct current of constant strength, and practically neutralizing the magnetizing effect of the line-current upon the motor at a maximum speed predetermined for all loads, by the sudden generation of a neutralizing-current near this speed, substantially as described.

3. The method of keeping the speed of a series-wound direct-current motor upon a constant-current-supply circuit between fixed limits, which consists in fixing the minimum speed of the motor by a maximum load, and the maximum speed of the motor by practically neutralizing the magnetizing effect of the line-current upon the, motor at a given maximum speed predetermined for all loads, substantially as described.

4. The method of keeping the speed of a series-wound direct-current motor upon a constant-current-supply circuit between fixed limits, which consists in fixing the minimum speed of the motor by a maximum load, and the maximum speed of the motor by suddenly practically neutralizing the magnetizing effect of the line-current upon the motor at a given maximum speed predetermined for all loads, substantially as described.

In testimony whereof I have signed my name to this specification in the presence of two subscribing witnesses.

MAURICE LEBLANC.

l/Vitnesses:

ARMANI) LEBLANC, AUeUs'rUs E. INGRAM. 

